The experiments are designed to provide new and basic information about what happens in certain neurons of the cerebral cortex after learning or after selected neural systems have been excessively stimulated. Experiments on adult cats have shown that portions of pyramidal cell neurons of the cortex probably form new growths as a result of increased activity of the cells. Quantitative data were obtained using a modification of the Golgi Cox histological method and the light microscope. New experiments on the same type of animal preparations will look for changes in synaptic elements using the electron microscope. With electrophysiological techniques the excitabilities and discharge properties of cortical neurons will be studied following conditioning and/or electrical stimulation of neural pathways leading to the cells. Other experiments involve current density and morphological studies of undercut neocortex with and without conditional training, using as the conditional stimulus electrical stimulation of the undercut cortex. A cat loses its acquired ability to give conditioned leg flexions when the conditionally excited cortex is undercut. However, the cat can eventually be retrained with the same electrical conditional stimulus. This observation seems to indicate that considerable reorganization has occurred in the undercut cortex so that intracortical transmission capability is enhanced or facilitated to the point where neuronal communication out of the slab is possible. A quantitative study of synaptic elements in "trained" and "untrained" cortical slabs will be done with the electron microscope. Preliminary data are encouraging. Finally, current density profiles will be made on undercut cortex ("trained" and "untrained") in an attempt to define critical cortical layers and neuronal elements that may be responsible for the altered intracortical transmission of neuronal activity. This approach has important implications for new concepts about modified conduction in other neural systems.